Research On Fabrication Technology And Diffraction Characteristics Of Fresnel Zone Plate With High Diffraction Efficiency

Compared to traditional reflective or refractive optical elements,diffractive optical elements have lots of advantages such as more design freedoms,looser fabrication tolerance and unique properties for imaging with flat plate.Diffractive optical elements are divided into imaging elements and non-imaging components.Imaging elements are widely applied to variety of wavelengths including ultraviolet,visible and infrared wavelengths.Non-imaging components are mainly used for energy collection,intensity homogenization and wave front correction.As the most common used diffractive optical elements,Fresnel zone plate is widely used in variety of optical systems.In the area of X ray imaging,since the refractive index of most materials is close to 1,as a result,it is unable to focus though the light deviation based on refraction theorem,in contrast,Fresnel zone plate based on diffraction can effectively overcome this problem,thus playing an irreplaceable role in x ray imaging.For telescopes used in space,in order to obtain higher resolution,it is common to increase the aperture of telescope or traditional reflective-type space telescope,however,larger aperture will introduce extremely stringent requirements for both optical fabrication and optical testing,besides,the volume and weight of the telescope becomes larger with the increase of aperture,which will bring great challenges for the launch process,in contrast,the Fresnel zone plate,which can be folded on the ground and expanded in space,turns out to be a preferable choice for the next-generation larger telescope.Low diffraction efficiency is the biggest drawback of Fresnel zone plate,restricting its application in lots of optical system.Light from the non-working order also focus on the focal plane as the background,which will decrease the signal to noise ratio,thus affecting the imaging quality of optical system.Considering these disadvantages,it is particularly important to undertake the research on how to increase diffraction efficiency combined with specific processing technology.During the fabrication process,different fabrication errors have an influence on the diffraction efficiency,so it is practically significant to set up a diffraction model to develop quantitative analysis of the effects of different processing errors on diffraction efficiency.In this paper,we have made deep research and exploration on the multi-step Fresnel zone plate with high diffraction efficiency based on both theoretical analysis and experiments,the main contents of the research paper includes the following aspects1.Study on the effect of phase level on diffraction efficiencyThe relationship between diffraction efficiency and number of phase levels is analyzed by scalar diffraction theory,under certain approximate conditions,according to Kirchhoff diffraction formula and principle of linear superposition,the analytical expression of optical field for both amplitude and phase-type FZP is derived.The diffraction efficiency of different type FZP are analyzed according to this formula.2.Study on the fabrication technology of multi-level Fresnel zone plate with large aperture and high precision.During the etching of large aperture Fresnel zone plate,the processing aperture as well as the etching uniformity are limited.In order to etch FZP with larger aperture,a new method to combine point source ion beam with dwell time algorithm is proposed.Using this method,the etching aperture is no longer limited by the size of ion source,elements with large aperture can be etched with this method through scanning.A 4-level FZP with the aperture of 300 mm is fabricated though overlay process,the testing optical system is set up and the diffractive optical properties are measured.3.Study on the fabrication technology and diffraction properties of hybrid-level Fresnel zone plate.On one hand,considering the diffraction efficiency,the bigger of the line width,the better it is,since it is easy to fabricated muti-level structure,it is very difficult to fabricate FZP with very small line-width because the minimum line width is small enough that beyond the fabrication capacity of laser direct writing system,on the other hand,considering the resolution,smaller line width is better.In order to get high diffraction and high resolution at the same time under a certain process capacity,a hybrid-level FZP is proposed to improve diffraction efficiency at the most extent.The diffraction model for hybrid-level FZP is set up to analyze its diffraction efficiency and imaging ability.A hybrid-level FZP with an aperture of 20 mm and a F number of 15 is fabricated,the diffraction efficiency and imaging property are measured.4.Study on the effect of fabrication error on diffraction efficiency.The model of fabrication error for multi-level FZP is set up.The effect of line width error,etching depth error as well as overlay error on diffraction efficiency is analyzed.During the fabrication process,it is the relative overlay error instead of the absolute overlay error that will influence the diffraction efficiency.The relative overlay error is changing because of the changing line width,so it is reasonable to analyze diffraction efficiency with consideration of relative overlay error instead of absolute overlay error.The concept of integrated efficiency is proposed,and the integrated efficiency of FZP with different overlay error is simulated.